A method for heat treatment or rail steels in which the complete rail or only the head portion of the rail is heated to austenitization temperature and thereafter cooled so rapidly that through transformation in the lower pearlite stage a fine lamellar pearlite structure is obtained, is known from "Stahl und Eisen", 1970, No. 17, page 926 and following. It is the objective of such thermal treatment processes to produce in rails presenting a guide analysis in compliance with class 90 A of UIC print 860-V, respectively according to AREA-standard (0.60 to 0.75% carbon, 0.80 to 1.30% manganese, up to 0.50% silicon) at the contact surface of the rails up to a depth of at least 10 mm a fine pearlitic structure. Said fine pearlitic structure provide increased resistance against wear and a four to six time higher service life compared to non heat treated rails.
In said methods the usual austenitization temperature is between 850.degree. to 900.degree. C. The heating is performed in a furnace by an electrical induction system, or by a gas burner. Rapid cooling is obtained by oil quenching or blowing of water vapor, water jet or compressed air.
The hardness values obtained by such a process range between 320 and 380 HV at the contact surface. Towards the middle of the rail head portion hardness drops in compliance with the heat treatment process progressively or abruptly to about 280 to 300 HV.
Such heat-treated rails are used on highly frequented rail sections, narrow curvatures and/or at axial loads of more than 200 kN or in switches.
Nevertheless, for particularly excessive loads the strength and wear resistance of the known heat-treated rails are not sufficient. An increase in strength by maintaining the desired fine pearlitic structure profitable for wear characteristics by admixture of strength increasiang alloy elements such as chromium, manganese, nickel and molybdenum is not possible since in the described heat treatment the admixture of these alloy elements causes a transformation partly in bainite and martensite instead of a transformation in the lower peariltic stage thus producing structures which have a negative influence on wear characteristics and resistance against rupture.
From "Technische Mitteilungen Krupp, Werksberichte", Vol. 37 (1979), pages 79 to 94 highly resistant pearlitic rail steels are known which have a fine grain structure with small interlamellar spacing and low thickness of the cementite and ferrite lamellas after admixture of a maximum of 1.4% chromium and up to 2% nickel. Compared to a steel comprising about 0.75% carbon and about 1% manganese a tensile strength of up to 1350 N/mm.sup.2 in natural hardness--air cooled--condition, an increase of said strength by heat treatment, for instance accelerated cooling into the area of the lower pearlite stage, leads to the above mentioned undesired portions of bainite and martensite in the structure.
It is an object of the present invention to further develop a method for heat treatment of rail steels of the above type. Maintaining the fine lamellar pearlitic structure at the rail contact surface hardness values shall be obtained of more than 380 HV, said hardness still reaching values of more than 360 HV in a depth of 15 mm below the rail contact surface.